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| Habilitation, Postdoctoral Thesis | PreJuSER-37550 |
2005
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/254
Report No.: Juel-4158
Abstract: The concept of a cold radiating plasma boundary has been proposed as a solution of the problem of power exhaust in magnetically confined fusion plasmas. In the tokamak TEXTOR the injection of impurities (neon, silicon or argon) leads to the formation of a radiating plasma boundary where up to 90% of the input power can be distributed to large wall areas, thereby strongly reducing the convective heat flux density onto the plasma facing components. At high plasma densities the impurity seeding leads to a transition to an improved confinement state, termed the Radiative Improved Mode (RI-mode). This operational scenario combines high density and high confinement with power exhaust by radiation under quasi-stationary discharge conditions. The plasma density can be further increased by external gas fuelling, while the high confinement is maintained, if the gas is fuelled at a moderate rate. In contrast, strong gas fuelling leads to a confinement degradation back to the normal L-mode level. We have used plasma diagnostics based on optical methods to characterise particle, energy and neutral transport at the plasma boundary. When the radiated power is increased owing to the injected impurities, we observe a strong reduction of the plasma temperature at the plasma boundary. At the same time, the particle transport out of the confined volume decreases, accordingly the neutral flux back into the plasma decreases. In discharges, where the density is increased by strong gas fuelling and the energy confinement degrades, we observe a built-up of edge density and neutral pressure owing to a higher recycling coefficient. In contrast, with moderate gas fuelling the edge density and neutral pressure remain low. While the seeded impurities cause a substantial dilution at the plasma edge, the plasma core is much less affected, leading to hollow impurity concentration profiles. Operation at the highest densities is favourable with respect to the release of impurities at the edge as well as with respect to the resulting impurity content in the plasma bulk. The most prominent change of transport in the plasma core is a steepening of background density profile while the temperature in the core can be maintained or even slightly increased. The global energy confinement is strongly correlated with the density peaking. An analysis of the experimental plasma profiles with respect to stability against the ion temperature gradient driven mode (ITG mode) shows that the ITG mode is substantially reduced during the confinement improvement and that it reappears if the density is increased and a subsequent confinement roll-over occurs with too a strong gas fuelling. The dynamics of both the confinement improvement and of the degradation is initiated at the plasma edge. The resulting changes in the plasma core amplify the initial trigger. Consequently, we observe a non-linear interplay between edge and core which allows for a self-organisation of the plasma and a bifurcation between two rather different states. The transition and the changes of the plasma profiles can be described in agreement with the experimental findings by a transport model based on the ITC mode and the dissipative trapped electron (DTE-) mode, which is connected to a strong anomalous particle pinch. This inward pinch leads to the steepening of the density profile once the ITG mode growth is reduced by the impurities. During the confinement degradation with strong gas injection increased edge transport leads to a reduction of the impurity content in the core below the level needed for the ITC suppression.
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